Central and Northern Georgia received substantial levels of rainfall from September 17 to 22, 2009. Parts of Northeast Atlanta received over 13 inches of rainfall while areas such as Canton, Marietta, and Kennesaw received 18 to 20 inches. Many homeowners are faced with tremendous flood damage to their property including landscaped areas and turfgrasses. Landscapers are receiving questions from their clients concerned about loss of fall preemergence herbicides applied prior to the flood.

Preemergence herbicides are tightly bound to the soil and are generally not readily leached by water. If preemergence herbicides are irrigated into the soil after application or rainfall is received within a few days, these herbicides are activated and moved off of the surface and in the upper soil layers. Location near the weed seedbank and binding of preemergence herbicides to soil is critical for annual weeds to absorb these materials after germination. With the tremendous amount of rainfall recently received in Georgia, turfgrass managers are concerned about potential loss of preemergence herbicides applied prior to the flood.

Generally, preemergence herbicide loss in soil is from soil microorganism activity while leaching, or movement with water, is minimal. Flooding does have potential to move preemergence herbicides remaining on the turf canopy off site if applications were not properly irrigated into the soil. Preemergence herbicide loss may also occur if soil erosion resulted after the flood by physically removing soil with the herbicide. Another potential loss of preemergence herbicides is from anaerobic (in the absence of oxygen) degradation. Loss of preemergence herbicides can accelerate when there is a shift in microbial populations from aerobic (in the presence of oxygen) to anaerobic microbes. Under waterlogged conditions, there is limited oxygen exchange and anaerobic microorganisms increase. They can use preemergence herbicides as a food source, thus, breaking down the herbicide and resulting in diminished weed control. This scenario becomes a concern when water remains on a lawn for three to four days. If a lawn, sports field, or golf course was briefly submerged and the waters receded once the rainfall ceased, anaerobic breakdown is not likely an issue.

Lawn Care Companies who applied preemergence herbicides before the flood to areas without erosion will likely not need to reapply these products. However, making a sequential application of moderate rates of these herbicides after six to eight weeks may help extend soil residual control of winter annuals, especially if herbicide loss is a concern. Preemergence herbicides may need to be reapplied to areas with significant soil loss, damage to the upper layer of the soil profile, or incidences where the lawn was submerged for greater than four days. Contact your Weed Pro for further consultation with managing turfgrass and landscape issues from flood damage.